The depletion and regeneration of dissolution-active sites at the mineral-water interface:: I. Fe, Al, and In sesquioxides

Abstract

The responses of α-Al 2O 3 and In 2O 3 dissolution rates to pH-jumps in flow-through reactors are compared to previous results for α-Fe 2O 3 in order to test the following hypothesis: because the same fundamental processes of ligand exchange and detachment of metal centers govern both steady state and nonsteady state dissolution, both the steady state and nonsteady state dissolution rates of these sesquioxides correlate with the water exchange rates of the corresponding aqueous ions. Our results show that steady state dissolution rates at pH 1 for α-Fe 2O 3, α-Al 2O 3, and In 2O 3 correlate with the rates of water exchange. Nonsteady state dissolution rates, in response to jumps to pH 1 from higher initial pH values at which the oxides had been aged, also correlate with water exchange rates although in a complicated way. The amount of solute dissolved during nonsteady state dissolution increases with increasing initial pH, and is consistent with the depletion at low pH and regeneration at higher pH of a reservoir of surface sites active for dissolution at pH 1. This suggests that the oxide surfaces must be sufficiently kinetically dynamic at circumneutral pH for processes to occur that result in the creation of dissolution-active sites. The pH dependence of the amount of solute released can be related to rates of active site production and dissolution, using the idea that hydroxylation of the mineral surface with increasing solution pH has a labilizing effect on the rates of ligand exchange at a surface metal center similar to the labilizing effect of hydrolysis on the rates of water exchange for aquaions.